Presentation on theme: "4.3 How Atoms Differ Atomic Number - Atomic Number = # protons in an atom - # of protons determines kind of atom (atoms of an element always have the."— Presentation transcript:
4.3 How Atoms Differ
Atomic Number - Atomic Number = # protons in an atom - # of protons determines kind of atom (atoms of an element always have the SAME number of protons) - The atomic number = the number of electrons in a neutral atom - Written just above the atomic symbol on the periodic table
Complete the Table: ElementAtomic ## Protons# Electrons 6 N Pb 3 19 Br
Symbols If an element has 91 protons and 140 neutrons what is the……..? –Atomic number –Mass number –number of electrons –Complete symbol
Symbols If an element has 78 electrons and 117 neutrons what is the ………? –Atomic number –Mass number –number of protons –Complete symbol
Naming Isotopes Put the mass number after the name of the element Put the mass number after the name of the element carbon- 12 carbon- 12 carbon -14 carbon -14 uranium-235 uranium-235
Mass of the atom Atomic mass on the periodic table is NOT an even # The atomic mass of an element is the weighted average mass of the isotopes of that elementatomic mass Depends on abundance of each isotope
Atomic Mass - How heavy is an atom of oxygen? - The MASS NUMBERS we’ve been using are measured in amu - 1 amu (atomic mass units) is ABOUT the mass of 1 proton or 1 neutron (or 1/12 of a carbon -12 atom) - On the periodic table, the MASS of each element is NOT reported as a whole number… - We can’t have HALF a proton or HALF a neutron in an atom… so why aren’t these masses whole numbers??
Atomic Mass - - Chlorine-35 is MORE common in nature than chlorine-37…so the mass of 1000 Cl atoms will be CLOSER to amu - - To take the average of the two isotopes, we need to use the WEIGHTED AVERAGE: - - In nature, 75.53% of Cl atoms are chlorine-35, and % are chlorine- 37… - - (% is called % abundance)
Calculating the WEIGHTED AVERAGE - - Write each percent as a decimal - - Multiply that decimal by the isotope’s mass - - Find the average by adding together Average = % as decimal x mass + % as decimal x mass - Average Mass of a Chlorine atom: (0.7553)(35) + (0.2447)(37) = amu (0.7553)(35) + (0.2447)(37) = amu % abundance isotope 1 Mass isotope 1 % abundance isotope 2 Mass isotope 2
Atomic Mass: EXAMPLE - Calculate the atomic mass of copper if copper has two isotopes. 69.1% have a mass of amu and the rest have a mass of amu…
ANSWER… Mass of Cu = (.691)(62.93) + (.309)(64.93) = amu
Atomic Mass: EXAMPLE 2 - Magnesium has three isotopes % magnesium-24, 10.00% magnesium-25, and the rest is magnesium-26 amu. What is the atomic mass of magnesium?
Answer… Mass of Magnesium = 7899 x 24) + (.1000 x 25) + (.1101 x 26) = ( x 24) + (.1000 x 25) + (.1101 x 26) = amu